1. Field of the Invention
This invention relates to a high-performance, reactionless, dynamic mount for a laser-beam steering mirror and especially to a motor for actuating a mirror supported by such a mount.
2. Description of the Prior Art
High-energy laser (HEL) systems generally employ an adjustable, cooled mirror to steer the laser beam. The mirrors are large and heavy, and beam control requires precise pointing, focusing and stabilization of the beam in a severely vibrational environment. To date, a mirror mounting-and-moving device capable of satisfactorily performing these control functions has not been demonstrated even though extensive effort has been expended thereon.
Perturbations arising from acoustic, thermal and maneuvering loads in the mirror, its mount and its dynamic actuator require the actuator (or motor) to be capable of providing several milliradians of stroke and to have an operating bandwidth from D.C. to several kilohertz (KHz). Stroke precision is on the order of a few microradians. The high intensity of the beam on the mirror surface and the low allowable distortion of the mirror surface combine with high dynamic loads to increase mirror size and weight (in excess of 30 lbs.).
Mounting a HEL in an aircraft or a space satellite places a premium on drive power component weight. Large values of actuator heat dissipation in the mirror dynamic mount contribute to thermal distortion and complicate thermal management. Additionally, it is desirable to minimize disturbances in the supporting optics structure, arising from the large reaction forces in the dynamic devices.
Actuators formed from stacks of piezoceramic wafers have been used for some time to control the surface orientation of laser mirrors or as dynamic drivers for small mirrors and elements of mirror arrays. These actuators utilize strains imposed by an electric field in the direction of thickness (d.sub.33 direction) or length (d.sub.31 direction). Actuators of this type are limited to 200-300 microinches of movement per inch of their length even if operated at a high voltage (e.g., 30-50 volts/0.001 inch of wafer thickness).
In an operating system, the dynamic mirror mount actuators are driven be a "closed loop", electronic servo system. Error signals may be obtained from a variety of optical sensor arrangements depending upon specific system considerations. In all cases, however, stability of a practical servo drive requires that structural resonances of the actuator drive train be higher than operating frequencies. Control of heavy heat exchanger-type mirrors (cooled mirrors) at several kilohertz requires actuator spring rates of tens of millions of pounds per inch of deflection.
Piezoceramic actuators appear basically as pure capacitive loads to the driving amplifier. As a consequence, almost all of the driving power is dissipated in the output stage of the amplifier. This is an advantage in thermal management of the actuators but places heavy burdens on amplifier design. The quantity of driver-dissipated, or "reactive power", is a function of the square of the driver output voltage for a given operating frequency. Reactive power also increases in proportion to frequency. For example, if an actuator requires 100 volts to achieve a required displacement at a frequency of 300 Hz, and this corresponds to a reactive power of 200 watts, the reactive power would increase to 20,000 watts if the required voltage were 1000 volts. Thus, lower operating voltages are desirable for piezoceramic actuating motors.
As described, the conventional piezoceramic actuator requires excessive length to achieve required deflections, the resultant structural proportions resulting in excessively low resonance characteristics. Practical power considerations preclude high-frequency operation at high voltage. The unique piezoceramic shear motor used in the present invention provides a displacement per volt which is six times as much as that provided by a conventional PZT actuator. The block-like proportions are ideally suited to high structural resonance design. Also, the configuration used is inherently reactionless.